This invention relates to the covering, e.g. for connection and/or encapsulation purposes, of substrates and, more especially, relates to the formulation of an assembly comprising a substrate and a covering which is hermetically sealed around the substrate. The invention is especially applicable to heat-shrinkable covering articles used in the insulation and connection of, for example, pipelines and electric cables.
Heat-recoverable articles, which have been deformed from an original configuration and which are capable of returning towards that configuration on the application of heat, are well known and are generally made from polymeric materials, especially cross-linked polymeric materials, such, for example, as those described in U.S. Pat. Nos. 2.027,962 and 3,086,242, the disclosures of which are incorporated herein by reference. More recently, heat-recoverable articles have also been made from certain alloys, sometimes called "memory metals", as described, for example, in U.S. Pat. Nos. 3,174,851 and 3,351,463, the disclosures of which are also incorporated herein by reference.
One of the main uses of these heat-recoverable articles, especially when made from polymeric materials such as cross-linked poylethylene, has been in the field of electrical insulation and, more especially, in the connection and encapsulation of electrical conductors such as wires and cables. They have also been extensively used in the insulation and connection of service lines such as pipelines.
In these applications the heat-recoverable article usually in the form of a heat-shrinkable tube or a wrap-around sleeve, as described, for example, in U.S. Pat. Nos. 3,243,211; 3,297,819 and 3,379,218 and British Pat. Nos. 1,155,470; 1,211,988 and 1,346,479, respectively, the disclosures of which are also incorporated herein by reference, is caused to shrink about the substrate(s) being insulated or connected. In most of these applications it is necessary that a good seal is made between the ends of the heat-recoverable article and the substrate as protection against the ingress of moisture and small insects etc.. For this reason the heat-recoverable articles are commonly provided with an inner layer of adhesive or other sealant, e.g. a mastic, at least at their terminal portions. However, even then it is difficult to ensure that a reliable seal is obtained in all cases upon recovery and, more particularly, it is difficult to determine afterwards whether or not a moisture- and insect-proof seal has in fact been obtained.
The formation of a hermetic seal is, of course, also important when other forms of covering are used to connect or encapsulate the substrates. For example dimensionally-recoverable coverings may be employed in which conventional elastomeric recovery is used, including dimensionally-recoverable articles which comprise an elastomeric outer tube which is held in a stretched state by an inner tube which, upon heating, weakens and thus allows the elastomeric outer tube to recover.
In all of the above instances the heating used during formation of the covered assembly leads to a build up of air pressure within the covering (which may in some cases lead to disruption of the seal).